The present invention relates generally to a fixture of a board, and more particularly to fixture of a package board having a heat radiator mechanism. The present invention is suitable, for example, for a fixture between a heat sink and a Ball Grid Array (“BGA”), a fixture between a heat sink and another package, such as Land Grid Array (“LGA”) and Pin Grid Array (“PGA”). The present invention is also relates to a printed circuit board, such as a motherboard, mounted with such a package board, and an electronic apparatus, such as a server, equipped with the printed circuit board.
The recent developments of electronic apparatuses have increasingly demanded supplies of sophisticated and inexpensive electronic apparatuses. The BGA package has conventionally been proposed in order to meet this demand. The BGA package is one type of package board soldered to a printed board (which is also referred to as a system board and a motherboard). The BGA package realizes a narrower pitch and more pins (i.e., high-density leads) without enlarging the package size than a Quad Flat Package (“QFP”) that has the Gullwing type leads on the four sides. Thus, the BGA package enhances the performance of and miniaturizes the electronic apparatus through the high density of the package.
The BGA package is mounted with an IC and an LSI that generally serve as a CPU, and the improved performance of the CPU swells the calorific value. Thus, in order to thermally protect the CPU, a radiator called a heat sink is thermally connected to the CPU via a heat spreader. The heat sink has cooling fins, is located near the CPU, and radiates the CPU through natural cooling.
Referring now to FIG. 12, a description will be given of the conventional BGA package. Here, FIG. 12 is a schematic sectional view for explaining a conventional BGA package 1000. As shown in FIG. 12, a ceramic package board 1400 that is equipped with an LSI 1100 via bumps 1200 and underfill 1300 is mounted on a printed board 1600 via BGA 1500. A heat sink (not shown) is thermally connected via a lid-structured heat spreader 1700. The LSI 1100 and the heat spreader 1700 are adhered to each other via a joining layer 1800.
The conventional BGA package 1000 thus mounts the LSI 1100 onto the ceramic package board 1400, because the LSI 1100 and ceramic have similar coefficients of thermal expansion enough to prevent the LSI 1100 and the package board 1400 from warping in mounting the LSI 1100. Although the package board 1400 directly contacts the underfill 1300, a difference of a coefficient of thermal expansion between the LSI 1100 and the package board 1400 is dominant due to a small thickness of the underfill 1300. This structure uses the package board 1400 and the LSI 1100 having almost the same coefficient of thermal expansion, and maintains very small the stress associated with the thermal expansions and contractions.
The heat spreader 1700 is adhered to the back of the LSI 1100 via the joining layer 1800. Even when the heat spreader 1700 is made of a material with a high thermal conductivity, such as Cu, the entire package cannot improve the heat transfer efficiency, because the joining layer 1800 needs such a joining material as resin and silicon adhesive agents having a low thermal conductivity or as a sheet or paste joining material, causing a temperature gap. While it is conceivable to use metal having a high thermal conductivity, such as solder, for the joining layer 1800, a difference of a coefficient of thermal expansion between the LSI 1100 and the heat spreader 1700 causes a strong thermal stress between them as the temperature of the LSI 1100 rises, causing damages of the joining layer 1800 and/or the LSI 1100.
Accordingly, use of liquid having a high thermal conductivity, such as liquid metal, for the joining layer 1800, is proposed to eliminate the thermal stress that would otherwise occur between the LSI 1100 and the heat spreader 1700, and to provide a BGA package having a high thermal conductivity (see, Japanese Patent Application, Publication No. 60-84848). FIGS. 13A to 13C are schematic sectional views for explaining a BGA package 2000 that uses the liquid metal for the joining layer. The BGA package 2000 is manufactured by etching an LSI 2100 shown in FIG. 13A to form a concave 2200 as shown in FIG. 13B, and then by injecting liquid metal 2300 into the concave 2200 and sealing the liquid metal 2300 by a heat-conductive coating 2400 that completely separates the liquid metal 2300 from the air and substrate, because the chemical characteristics of the liquid metal 2300 is likely to cause chemical reactions and erosions, such as hydroxylation and oxidation.
Another prior art proposes a BGA package structure that brings the top surface of the LSI into contact with the coolant and circulates the coolant.
For higher performance of the BGA package, use of resin for the package board instead of ceramic is studied. The resin board is thinner than the ceramic board, and expected to have a more improved electric characteristic than the ceramic board.
However, due to a difference of coefficient of thermal expansion between the resin package board and the LSI, the thermal stress occurs between them as the LSI's temperature rises. In particular, a difference of coefficient of thermal expansion between the resin package board and the LSI is so big that the LSI warps and the heat spreader adhered to the LSI's back surface also warps subject to the influence of warps between the LSI and the package board. Then, as shown in FIGS. 14A, 14B, and 15, the joining layer 1800 peels off, the LSI 1100 and the heat sink (not shown) are thermally disconnected to each other, and the LSI 1100 gets thermally damaged. Of course, physical damages are likely to occur in the LSI 1100, the joining layer 1800, and the heat spreader 1700 due to the warps of the LSI 1100. Here, FIGS. 14A and 14B are schematic sectional views of the BGA package 1000 for explaining the prior art problems, wherein the LSI is at a high temperature in FIG. 14A and the LSI is at a low temperature in FIG. 14B. FIG. 15 shows an enlarged sectional view showing a thermal disconnection between the LSI and the heat spreader due to the peeling off of the joining layer.
In addition, the BGA package that uses the liquid metal for the joining layer requires the fine processing technologies, such as etching, and high processing technologies, such as forming of the heat-conductive coating and sealing of the liquid metal. The heat-conductive coating as large as the LSI has no heat dispersion capability and the improved cooling capability for the LSI cannot be expected while its calorific value increases as its performance improves. Moreover, as described above, the difference of coefficient of thermal expansion between the resin package and the LSI may cause warps in the LSI and damages of the heat-conductive coating. An expansion of the liquid metal may also cause damages of the heat-conductive coating. One conceivable solution for the BGA package 2000 is to seal the liquid metal 2300 with the heat-conductive coating 2400, enclose the LSI 2100 with cooling fins 2500, and mount the integral structure onto a package board 2600, alleviating the damages of the heat-conductive coating 2400. However, this structure becomes big and complex. In addition, a joint between the heat-conductive coating 2400 and the heat sink 2500 remains problematic as to the thermal conductivity.